Question

Relative afferent pupillary defect (RAPD) is characteristically seen in damage to:

• A. Optic nerve
• B. Optic tract
• C. Lateral geniculate body
• D. Oculomotor nerve

Hint:

RAPD is an input-side defect found with the swinging flashlight test; which structure carries that afferent signal?

Answer 1

The Correct Option is A

Step 1: A relative afferent pupillary defect (RAPD), detected by the swinging flashlight test, reflects asymmetry in the AFFERENT (sensory) limb of the pupillary light reflex between the two eyes.
Step 2: The afferent limb runs from the retina along the optic nerve; light input from each eye is then shared (decussated) so both pupils normally constrict equally to light in either eye.
Step 3: A unilateral optic nerve lesion (for example optic neuritis) reduces light signal from that eye. When the light swings to the affected eye, both pupils paradoxically dilate because the afferent input is weaker. This is the classic RAPD. So Option A is correct.
Step 4: The oculomotor nerve carries the EFFERENT limb. Its damage causes a fixed dilated pupil on that side, not a relative afferent defect, so Option D is wrong.
Step 5: Because of the extensive crossing of fibres, lesions of the optic tract or lateral geniculate body produce visual field defects but typically not a clinically obvious RAPD as seen with optic nerve disease; so Options B and C are not the characteristic site.
Hence the answer is Option A (Optic nerve).